The present invention relates in general to radar absorbing materials, particularly broadband electro-magnetic energy absorbers. More particularly, the present invention relates to an electromagnetic energy absorber that is thin, flexible, lightweight, and preferably operates in a frequency band of 2-18 GHz with less than -15 dB reflectivity. Even more particularly, this invention relates to an improved absorber element configuration with improved broadband and reflectivity characteristics and attendant suppression of grating lobe signals.
Two basic forms of radar absorbers are referred to in the prior art as a Salisbury screen and a Dallenbach layer. The Salisbury screen is a resonant absorber formed by placing a resistive sheet on a low dielectric constant spacer in front of a metal plate. The Dallenbach layer consists of a homogeneous lossy layer backed by a metal plate. The Salisbury screen has found some limited usage, but is generally ineffective for broadband applications. One of the problems with the Dallenbach layer is the difficulty in providing the proper match of materials. Also, the Dallenbach layer does not provide sufficient bandwidth.
Much effort has been carried out in the past in an attempt to extend the bandwidth of radar absorbers through the use of multiple layers. In this regard, see by way of example, U.S. Pat. No. 2,951,247 to Halpern, et al, U.S. Pat. No. 2,992,425 to Pratt and U.S. Pat. No. 2,771,602 to Kuhnhold. Also refer to British patent 665,747.
In these prior art absorbers, the intention of the use of multiple layers is to slowly change the effective impedance from free space to zero ohms with distance into the material so as to minimize reflections or to provide an input impedance that matches that of free space as closely as possible over a selected range of frequencies. There are, generally speaking, two different types of multi-layer absorbers that are common in the art. These are referred to as the Jaumann absorber, and graded dielectric absorber. All of these absorbers require the use of multiple layers and are typically relatively thick. Existing broadband radar absorbing materials require thickness of at least one or two inches to achieve any significant bandwidth. Also, the manufacturing process is relatively complex because of the multi-layering of different materials that are used to obtain the broadband enhancement. One example of a commercially available graded dielectric absorber is one made by Emerson & Cuming. This is referred to as their Model No. AN-74 which is a three-layer foam absorber that is over one inch thick.
Accordingly, it is an object of the present invention to provide an improved radar absorbing material that has excellent broadband characteristics and that is yet thin, preferably flexible and light in weight.
Another object of the present invention is to provide an improved radar absorbing material that is in particular usable over a frequency range of 2-18 GHz with preferred reflectivity of less than -15 dB.
A further object of the present invention is to provide a radar absorber that is relatively simple in construction and that can be easily manufactured in production quantities at relatively low cost.
A further object of the present invention is to provide an improved radar absorber in which the overall material thickness is made quite small by employing a process that includes the step of printing antenna patterns using a preferred resistive ink and wherein the antenna patterns may be printed using silk screening techniques.
Another object of the present invention is to provide an improved radar absorber that is characterized by its broadband absorption, and yet is carried out with a thin structure at least an order of magnitude thinner than one inch.
A further object of the invention is to provide an improved radar absorber that is in particular adapted for high temperature applications.
Still another object the present invention is to provide an improved electromagnetic energy absorber that is in particular characterized by an improved absorber element configuration that provides improved broadband and reflectivity characteristics along with attendant suppression of grating lobe signals.
A further object of the present invention is to provide an improved absorber element configuration in accordance with the preceeding object and in which the absorber elements are disposed are randomly absent any substantial alignment of elements so as to prevent grating lobe enhancement occasioned by periodicity of placement of the absorber elements.